Unbreak parallel builds (-jX) on recent -CURRENT, where our make(1) is
bmake(1) now. The build fails because bmake(1) treats MFLAGS variable
slightly differently: while by default its value would be '-j X', our
old make(1) allows to set it from the makefile or environment, bmake(1)
will append to it instead. Pass the empty value via MAKE_ARGS to make
every make(1) happy, and thus fix parallel builds (once again).